Conspicuity for signage is determined by the contrast between the sign and its background. A sign must be conspicuous first, because, without it, the sign’s legibility and readability are moot points. While the appropriate size for signs is addressed on this website under the heading “How big should a sign’s letters be?”, conspicuity includes factors that only indirectly relate to size.
Anything that impedes the ability to detect a sign impacts its conspicuity: Trees, telephone poles, parked trucks and other signs are physical objects, and other factors include lack of illumination at night, driving west at sunset or east at sunup.
The two types of conspicuity are search and attention. Search conspicuity occurs when the motorist is actively looking for a gas station, food, lodging, etc. Attention conspicuity involves unexpectedly important signage, such as the presence of “lanes closed ahead” signs. Generally, signs are three times more likely to be seen in search mode than attention mode.
“Cone of vision” is another major factor for conspicuity. The Southern California College of Optometry determined, for a motorist, their peripheral vision while still looking straight ahead while driving, is a maximum of 10 degrees left and right. Consequently, a sign’s distance from the road, called the “setback,” is absolutely critical in terms of conspicuity.
For example, at the same 10-degree viewing angle, a sign that’s 42 ft. from the road “disappears” from view when the driver is closer than 240 ft. If the sign is only 5 ft. from the road, at the same viewing angle, it remains in the driver’s sight until within 30 ft.
The Larson Institute of Penn State University, from 10 studies it conducted between 1996 and 2010, calculated a formula for determining when signs come into view in relation to setback: L = D x 0.176, where L equals 10 degrees of “lateral offset” and D is the distance in feet of the sign at initial detection. Thus, if the initial detection distance from the sign is 300 feet, 10 degrees of lateral offset would be 52 ft. Note that this offset is from the driver’s eye position, and not from some variable point. This is included in a 2015 publication called the United States Sign Council Best Practice Standards for On-Premise Signs. Go to http://www.ussc.org/pdf/USSCSignStandardsJune102015FINALEDITION.pdf.
Similarly, Penn State has calculated how tall a sign should be, based on the distance from which it would be viewed. Using five degrees of vertical elevation, plus 3.5 ft. representing elevation of the average driver’s eye position above the road, a calculation of vertical sign-height limits, capable of providing comfortable detection over both long and short ranges, can be derived from the following equation:
H = D x .088 + 3.5, where H equals the sign-height minimum and D equals the distance in feet from the sign at initial detection. This is also included in the 2015 USSC publication.
Thus, if the initial detection distance from the sign is 400 ft., the sign height must be at least 38.5 ft.
Signs that are perpendicular (facing) to the motorist, such as freestanding and projecting signs, are significantly more conspicuous than signs that are parallel to the road, such as fascia signs or letters attached to a building.
Penn State studied this phenomenon and found that 50-60% of the parallel signs weren’t even seen by motorists, even if they were three times bigger than corresponding perpendicular signs. Overall, perpendicular signs are generally four times more conspicuous than parallel signs.
Penn State has also calculated to what extent signs are blocked from view at specific distances, based on both the number of lanes of traffic, the amount of traffic and the sign’s setback. Numerous tables are included for the variables in the 2015 publication: which of the four lanes the motorist is in, the speed of travel, etc.
In a worst-case scenario, when a driver is in the curb lane of a four-lane highway, and the traffic flow is 1200 cars per hour, and the sign is 10 ft. off the road, a sign might be blocked from 77% of the drivers.